1. Field of the Invention
The present invention relates to high power microwave antenna feeds and to techniques for fabricating same.
While the invention is described herein with reference to a particular embodiment for a particular application, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings of the present invention will recognized additional modifications and embodiments within the scope thereof.
2. Description of the Related Art
Sinuous or folded waveguide line feeds have been developed for high power microwave applications. Often referred to as "serpentine" feeds, these devices provide a low cost technique for feeding power to large planar arrays such as those used for land and ship based radar antennas.
As discussed in Radar Handbook, by Merrill I. Skolnik, published by McGraw Hill Company, 1970 and in the Final Report of Hughes entitled NOSC CR 219 (this report is subject to export controls), a serpentine feed is a long transmission line which is folded for space considerations giving it a serpentine shape. The line is tapped at periodic intervals to provide preselected amounts of power.
Within the serpentine is a primary (or main) channel through which the microwave energy passes. In addition, a number of secondary channels are coupled to sections or elements of the serpentine to provide output coupling. Typically, the coupling is provided by a plurality of slots in the main wall of the serpentine.
Initial attempts to fabricate serpentine feeds involved alignment of slots in the walls of the secondary waveguide with matching slots in the walls of the primary waveguide. This was problematic not only because of the difficulty associated with alignment, but also because, invariably, there were gaps between the walls. At high power levels, the gaps caused undesirable arcing and losses which degraded the performance of the system.
Therefore, the currently favored dip brazing technique was developed by which the otherwise slotted wall of the secondary channel is cut away and the shell of the secondary is brazed to the main waveguide. As disclosed in the Hughes Final Report, supra, dip brazing involves the application of a brazing material to the edges of the alloys to be brazed. The brazing material, typically aluminum or aluminum paste, acts as a bonding agent. The alloy and bonding agent are subjected to a number of heating stages as a prelude to a final heating in a bath, such as molten salt. The alloy is heated until it the agent melts and flows to form the brazed bond. At this point, the alloy is typically in a plastic state.
Despite its current popularity, there are numerous shortcomings associated with dip brazing:
(1) The secondary waveguide is typically brazed to the main waveguide at the narrow sidewall. This inhibits the use of broadwall-to-broadwall couplers which offer high performance. One such coupler is the Riblet-Saad coupler. (See "Directional Coupler Design Nomograms," by Tore N. Anderson, in The Microwave Journal, May 1959, pgs. 34,38.) This class of coupler has superior control of amplitude and phase over a wider bandwidth than do broadwall-to-sidewall or sidewall-to-sidewall couplers. The broadwall-to-broadwall coupler also permits a more compact serpentine design.
(2) The secondary waveguide structure is weakened by the removal of a side wall. Attempts to remove less of the wall have proved to be expensive with limited success. This increases the susceptibility to stress of the secondary waveguide.
(3) The dip brazing process is stressful for both structures because the brazing occurs near the melting point and there are often temperature variations within the bath. The stresses may cause deformations and distortions in the waveguides which introduce losses.
(4) The brazed seams are difficult to hold dimensionally and it is impractical to visually inspect the critical internal dimensions of brazed serpentines. As a result, the seams may be nonuniform causing additional insertion losses, higher voltage standing wave ratios (VSWR) and cumulative random phase errors.
(5) Dip brazed surfaces can take on a matte finish. These rougher surfaces produce significantly higher insertion losses in very high power systems.
(6) There are typically a multitude of pieces in brazed serpentines. As a result, there is typically a buildup of tolerances making it difficult to hold to design parameters.
(7) The serpentine is susceptible to mechanical damage after brazing and before hardening. Heat treating is problematic because of the possibility of distortion.
(8) Finally, since the brazed serpentine is not a unitary piece of metal, there often exists a pressure differential between the sections of the waveguide. This causes deformations in the waveguide which adversely affect performance. This problem has been addressed in the past by the use of metallic or foam stiffeners between the sections. However, the use of these stiffeners adds both to the weight and the cost of fabrication.
While a number of the disadvantages of dip brazing may be overcome by machining the serpentine from a single block of metal, there are other problems associated with the closure of the serpentine waveguides and the machining of the coupling slots. Thus, there exists in the art a need to address the shortcomings of prior serpentine fabrication techniques.